1. Field of the Invention
This invention relates to braking control of induction motors, and more specifically relates to a method for implementing frictionless braking control of induction motors which relys on motor load sensing information and apparatus using the same.
2. Description of the Prior Art
Electronic braking of induction motors, i.e., frictionless braking, is well known. Electronic brakes of the prior art, such as the FASTSTOP.TM., manufactured by Electroid Co. of Springfield, N.J., and the SHORT*STOP.TM., manufactured by Ambitech Industries, Inc., of Westwood, N.J., utilize a method based on the injection of a controlled amount of direct current into stator windings of an alternating current (AC) driven induction motor to achieve braking after the AC has been removed.
Injecting direct current into a motor's stator windings after removing AC power induces a static magnetic field within the motor. The static field acts counter to the field generated within the motor as the rotor continues to spin after power is removed, i.e., a counter torque. The counter torque thereby stops motor rotation without mechanical braking means providing an essentially frictionless braking mechanism. However, such prior art electronic braking is imprecise and cannot be used in many braking control applications.
FIG. 1 depicts a conventional electronic braking circuit, softbrake circuit 2, the operation of which is based on the injection of direct current. An induction motor 4 is shown in the figure electrically connected through a first electrical node 6 to a motor control section 8. Motor control section 8 is electrically connected to an AC source 12 through a second electrical node 10.
The softbrake circuit 2 includes a contact switch 14 which is electrically connected to AC source 12 and motor control section 8 through electrical node 10. Contact switch 14 also electrically connects node 10 to a DC power and control section 16. DC power and control section 16 is also electrically connected to both a contact switch 18 and a logic and timing section 20. Contact switch 18 electrically connects DC power and control section 16 to both the motor 4 and the motor control section 8 through electrical node 6.
Logic and timing section 20 is shown in the figure electrically connected to a power sensing and switching section 22. Power sensing and switching section 22 is electrically connected through a third contact switch 24 to the motor control section 8. Power sensing and switching section 22 also provides an activation signal to the logic and timing section 20.
The softbrake circuit 2 operates in conjunction with motor 4 such that when motor 4 is powered, alternating current is delivered to the motor 4 through motor control section 8. Power sensing and switching section 22 senses the current flowing to the motor 4 through contact switch 24. When the driving current is removed, power sensing and switching section 22 generates an activation signal to activate the braking mechanism of softbrake circuit 2.
Logic and timer section 20 receives the activation signal from power sensing and switching section 22. Logic and timing circuits (not shown) within logic and timing section 20 respond to the activation signal by generating a DC control signal to DC power and control section 16. DC power and control section 16 responds by generating and supplying direct current to the motor 4 via contact switch 18 and electrical node 6.
The direct current generated by DC power and control section 16 is provided to the stator winding (not shown) of motor 4, inducing a static magnetic field therein. A rotor (not shown) within motor 4 rotating pursuant to existing motor inertia or torque generates a magnetic field which is acted upon by the induced static magnetic field. The static magnetic field attempts to align the field created by the rotating rotor thereby stopping rotation of the motor. The rotational or braking torque provided by softbrake circuit 2 is proportional to the field strength of the static magnetic field. The strength of the static magnetic field is directly proportional to the direct current injected into the stator windings of motor 4.
The above-described electronic braking mechanism provides no control mechanism for operating under varying motor inertial loads. Induction motors which operate with different torque requirements, i.e., varying motor inertial loads, require a more refined electronic braking control mechanism than is available in the prior art. For example, a single speed elevator motor operates with varied torque requirements in accordance with varying elevator car loads.
Single speed elevators typically utilize a very basic and inexpensive elevator control system to control the movement of elevator cars. The simplicity of single speed elevators and their controls make them extremely popular in buildings of eight floors or less. However, floor leveling problems are endemic within single speed elevator control systems. For example, stopping at a floor on either the ascent or descent is accomplished by merely removing power and applying mechanical braking at a fixed position above or below a floor. The elevator car subsequently glides to an uncontrolled stop. Such a system is inexact, rendering off-level and abrupt stops commonplace with varying elevator loads. Concomitantly, frequent user complaints as well as high maintenance cost are very common with such operation.
FIG. 2 shows a conventional single speed elevator control system. A hoist motor 26 is shown electrically connected to an AC power source 27 and an elevator control circuit 28. The hoist motor 26 is also mechanically connected to an elevator car 30 by a cable 29 extending over a rotatable pulley 31 and to a mechanical brake 25. An elevator car 30 is shown in FIG. 2 at a floor 32. A call station 34 and elevator car sensing means 36 are positioned at floor 32.
Call station 34 is electrically connected to a signal controller 39 and generates a signal transferred thereto indicating to which floor the elevator car must be sent. Signal controller 39 is electrically connected to elevator control circuit 28. Elevator car sensing means 36 is electrically connected directly to a relay controller 38. The relay controller is electrically connected the elevator control circuit 28 and generates and transmit relay control information thereto.
Operation of the conventional single speed elevator control system will now be explained.
A hall call signal may be transmitted by a user from call station 34 to summon elevator car 30. The hall call signal is received at signal controller 39. Signal controller 39 generates a control signal in response to the hall call signal and transfers the signal to relay controller 38. Relay controller 38 relays the control signal to elevator control circuit 28. Elevator control circuit 28 then generates the appropriate motor control signals to direct elevator car 30 to floor 32.
Hoist motor 26 is energized by AC power source 27 in response to the signal from relay controller 38. A number of buffer resistors (not shown) define the acceleration of the elevator car 30 when the hoist motor is powered. Varying the resistances of the buffer resistors varies the acceleration and, concomitantly, the smoothness of the elevator car start. The total resistance is defined manually by an elevator maintenance person, whose cost in labor adds to the elevator maintenance cost.
As elevator car 30 approaches the determined floor 32, a signal is generated at elevator car sensing means 36 indicating that the car 30 is within seven inches from the set stop position 32. Relay controller 38 responds to the generated car sensing signal by generating and relaying a signal to elevator control circuit 28. Elevator control circuit 28 causes the line voltage (driving power) to be removed from the hoist motor 26 in response thereto. Removal of power to the motor 26 also activates mechanical brake 25. Hoist motor 26 shuts down and the mechanical brake 25 mechanically stops the movement of elevator car 30. The stopping of elevator car 30 relies solely on the mechanical brake 25. The resulting stopped position of the car may be above or below the set stop position at floor 32, depending on a number of factors.
Several factors influencing the stopping accuracy of an elevator car stopped by mechanical means include the size of the load, i.e., the weight carried by the car, brake wear, brake tension and brake temperature. Conventional electronic braking mechanisms, i.e., frictionless braking, such as the braking circuit described above with response to FIG. 1, would probably improve the stopping accuracy for an elevator; however, known electronic braking system cannot provide precise control for exact stopping under various load conditions. Precise stopping of elevators is not only a requirement critical in minimizing maintenance costs, but also at times a requirement of local law.